A specific marker protein for olfactory receptor cells has been isolated from mouse and rat tissues. This marker protein has been used to prepare an antibody. By using the peroxidase-antiperoxidase immunohistological technique we have demonstrated the presence of the marker protein in the perikaryon region of mature olfactory receptor cells and in the entire length of axon to its termination in the olfactory bulb. In developing rats the protein is first demonstrable on the l8th embryonic day only in those olfactory receptor cells the perikarya of which are located most superficially in the epithelium; the entire length of some axons also contains marker protein. In mice this is true as early as the 14th embryonic day. In the latter species, this time corresponds to the stage of development when synapses between receptor cell and second order neurons are first demonstrable in the olfactory bulb. This suggests a relationship between marker protein synthesis and synaptogenesis in the primary olfactory pathway. Presently studies are under way to determine; 1) At what developmental age synapse formation between receptor cells and second order neurons occurs in rats and whether this age corresponds with the time when the marker protein is first demonstrable. 2) Localization by electron microscopy of olfactory marker protein in receptor cells. 3) Organ culture of combined explants of olfactory mucosa and olfactory bulb at various developmental stages. This will provide a model system that can be experimentally manipulated.